PROJECT SUMMARY Cancer related fatigue (CRF) is persistent exhaustion related to cancer and/or its treatment. CRF is the most severe during treatment and can severely interfere with activities of daily living. Pharmacologic agents to treat CRF have been studied but there is insufficient evidence to recommend their use. Non- pharmacological interventions for CRF have also been studied but are costly to implement and involve significant patient burden particularly among those in active treatment. Given the clinical impact of CRF, the goal of this project is to investigate a novel low-cost/low-burden intervention: systematic light exposure (sLE) to treat CRF. Two hundred forty-eight breast cancer (BC) patients undergoing adjuvant chemotherapy will be recruited from Mount Sinai Medical Center, Roosevelt Hospital, and Memorial Sloan Kettering Cancer Center and randomized to one of two conditions: bright white light (BWL) or dim red light (DRL), a common comparison condition in light studies. The BWL intervention consists of exposure to full spectrum bright light whereas the DRL condition will involve exposure to less intense red light (< 50 lux). Both groups will self-administer 30 minutes of light from identically appearing boxes daily throughout 4 cycles of chemotherapy. Outcomes will be assessed through standardized subjective and objective measures at eight separate time points. The study will specifically address recommendations made for interventions for CRF from the NCI Clinical Trials Planning meeting (JNCI,2013).The proposed study will be the first large multisite randomized control trial (RCT) to: 1) determine if sLE affects CRF among BC patients undergoing adjuvant chemotherapy; 2) investigate whether sLE improves sleep and depressive symptoms and normalizes circadian activity and diurnal cortisol rhythms; 3) explore the possible mediational roles of sleep, circadian activity/cortisol rhythms, and depressive symptoms 4) test and intervention that can be widely and inexpensively disseminated with no harm to patients; and 5) address areas of investigation identified by the NCI as a top priority. This RCT will have major